Conventional optical microscopes have a resolution limit which is determined by the aperture of the object lens, and a resolution better than about one-half the wavelength of the light used can principally not be achieved. In copending U.S. application, Ser. No. 06/563,722, filed Dec. 20, 1983, an optical near-field scanning microscope is disclosed which circumvents the resolution limit through the use of an aperture with an entrance pupil diameter that is small compared to the wavelength, and arranged at a distance from the object smaller than the wavelength. This microscope achieves a resolution on the order of one tenth of the wavelength, i.e., in the neighborhood of 50 nm.
Electron microscopes typically have resolutions of 20 nm vertical and 1 nm lateral, but their known disadvantage is that because of the high energies of the electron beam required in achieving a high resolution, most surfaces are severely damaged.
The scanning tunneling microscope of U.S. Pat. No. 4,343,993 operates with much smaller energies. Since its operation and structure is relevant in connection with the present invention, the brief description of the scanning tunneling microscope is in order.
A very sharp metal tip is raster-scanned across the surface to be inspected at a distance so small that the electron clouds of the atoms at the apex of the tip and on the surface area closest to the tip gently touch. A so-called tunnel current then flows across the gap provided a potential difference exists between said tip and the surface. This tunnel current happens to be exponentially dependent on the distance between tip and surface, and this phenomenon is used to generate a correction signal based on the deviations from a predetermined value occurring as the tip is scanned across the surface of the probe. The correction signal is used to control the tunnel distance so as to minimize the correction signal, and to be plotted versus a position signal derived from the physical position of the tip over the surface being inspected. This technique permits a resolution down to an atomic scale, i.e., individual atoms on a surface can be made visible.
The scanning tunneling microscope requires the existence of a potential difference across the tunnel gap. Accordingly, tunnel tip and surface to be inspected either have to consist of electrically conductive material or must be coated with such material. (An insulating surface layer thinner than the tunneling length is permissible.) Thus, the scanning tunneling microscope has a natural limitation where the surface of an insulator is to be studied. Obviously, many of its details are sacrificed if a surface must first be coated with a metal layer, however thin that layer may be.